1. Field of the Invention
The present invention relates to a displacement information detection apparatus and, more particularly, to an apparatus suitably applied to encoders (linear encoders and rotary encoders) which irradiate gratings attached to a scale (first substrate) and a substrate (second substrate) with a light beam which move relative to each other, and detects phase- or intensity-modulated signal light obtained therefrom, thereby detecting displacement information such as the position, position deviation amount, position deviation direction, velocity, acceleration, origin position, and the like associated with the scale and the substrate.
Also, the present invention relates to an apparatus suitably applied to an apparatus (a motor with an encoder or the like) which controls the current amount or direction of a driving apparatus such as an AC motor on the basis of such displacement information to rotate an object.
2. Related Background Art
Conventionally, an incremental type encoder is popularly used as an apparatus for measuring relative displacement information (displacement amount, velocity, acceleration, and the like) of an object with high precision. The encoder includes an apparatus for detecting origin information so as to calculate absolute position information.
In general, in an incremental signal detection mechanism in an encoder, a repetitive grating pattern of transparent and non-transparent (or reflection and non-reflection) portions is recorded on a relatively moving scale (first substrate), and a slit pattern (grating pattern) having the same pitch as the repetitive grating pattern is also recorded on a stationary slit substrate (second substrate). These substrates are stacked to be separated from each other by a gap G, and a collimated light beam irradiates the substrates. At this time, the transmission optical amount periodically changes in correspondence with the degree of coincidence between the patterns of the two substrates upon movement of the scale. The change amount at that time is detected by a light-receiving element to obtain an electrical sinusoidal incremental signal. Also, the signal is further converted into a rectangular wave signal by a binarization circuit to obtain an electrical incremental signal.
In an origin signal detection mechanism, a plurality of random grating patterns including transparent and non-transparent (or reflection and non-reflection) portions are recorded on a relatively moving scale, and the same random grating patterns are recorded on a stationary slit substrate. These substrates are stacked to be separated each other by a gap G, and a collimated light beam is irradiated onto the substrate. At this time, a signal light pulse having a maximum transmission optical amount is obtained at an instance when the patterns of the two substrates perfectly coincide with each other upon movement of the scale. The signal light pulse is detected by a light-receiving element to obtain an origin signal. Furthermore, the signal is converted into a rectangular wave signal by a binarization circuit to obtain an electrical origin signal.
On the scale and the slit substrate used for detecting relative displacement information, the grating patterns and origin patterns are juxtaposed. In many cases, an incremental signal and an origin signal are parallelly detected at the same time using a single optical system. In this case, both the detection principle of an incremental signal and that of an origin signal utilize the modulation effect of the transmission optical amount which depends on changes in degree of overlapping between the scale and the slit substrate.
Recent encoders are required to have high resolution in detection of displacement information. In order to detect an incremental signal with higher precision and higher resolution, grating patterns for detecting an incremental signal must be recorded at higher density. However, with this arrangement, the contrast of an intensity pattern tends to decrease owing to diffraction of light before the light is transmitted through the slit array on the scale and reaches that on the slit substrate.
In view of this problem, a method of reducing the gap between a scale substrate and a slit substrate (detection head) to several 10 .mu.m is known. However, when the two substrates are too close to each other, they may contact and damage each other due to errors a scale guide mechanism, and cannot be set so close to each other.
As a method of detecting an origin signal having a high resolution, a light beam is linearly condensed on an origin pattern track (ideally, a transparent/non-transparent pattern) arranged on a scale using a lens or a cylinder lens which is arranged in addition to optical elements used for detecting an incremental signal to illuminate an origin pattern consisting of a single slit aperture pattern formed on the scale. Changes in transmission optical amount from the origin pattern upon movement of the scale are detected, thereby obtaining an origin signal at a resolution as high as the detection resolution of an incremental signal.
However, a condensed light beam is required for detecting an origin signal. For example, a cylinder lens or the like must be inserted at a position near the slit aperture pattern between a collimator lens and the scale, and as a result, a size reduction and assembly of the apparatus become difficult.
Furthermore, since the cylinder lens is inserted, the detection timing of an origin signal deviates depending on the way of attaching the cylinder lens, and it becomes difficult to satisfactorily synchronize an origin signal with an incremental signal.